1) Field of the Invention
The present invention relates to an optical scanner used in an optical writing system, such as digital copying machines and laser printers. The present invention further relates to an image formation apparatus using the optical scanner. More particularly, the present invention relates to an optical scanner suitable for a multi-color image formation apparatus for forming a color image by superposing toner images of a plurality of colors, and an image formation apparatus using the optical scanner.
2) Description of the Related Art
In image formation apparatuses using the Carlson process, formation of latent images, development of the latent images, and transfer of the developed latent images are carried out with rotation of photosensitive drums. For example, multi-color image formation apparatuses include a plurality of photosensitive drums arranged along a transport direction of sheet media onto which images are transferred, and image forming stations for respective colors forming toner images which are to be superposed onto each other. In these image formation apparatuses, differences in time from which the latent image is formed to which the latent image is transferred caused by eccentricity of the photosensitive drums and variation in the diameters of the photosensitive drums, differences in distances between the photosensitive drums, and variation in speed or meandering of transfer members such as transfer belts or conveyor belt which carries the sheet media are generated. As a result, toner images become out of registration and thus colors of the images formed become out of registration or color change is caused. Therefore, image quality is degraded.
Likewise, in optical scanners, which forms latent images on photosensitive drums, if positions at which latent images are formed on the photosensitive drums do not precisely match each other, colors of the images formed become out of registration or color change is caused when the differently colored images are superimposed.
These deviations have been corrected by regularly, for example job by job, detecting positions in a sub scanning direction of detection patterns recorded on transfer members, and adjusting timings at which writing is started, without considering whether the deviations are related to the optical scanners or means other than the optical scanner (see the Japanese Patent Publication No. 3049606, and the Japanese Patent Publication No. 3078830).
Heat generated by the polygon motor or the like causes temperature differences between scanning lenses for different colors in between the corrections, and thus the refracting power changes. Consequently, colors of the images generated become out of color registration due to the deviations in the sub scanning positions and to differences in magnification in the horizontal scanning direction.
Examples in which displacement of scan lines caused by a machining error or thermal deformation of the scanning lens, a deviation in the height of beams incident on the scanning lens, or so-called decentering is corrected have been disclosed. One of the examples in which the displacement is corrected by shifting the scanning lens having a power in the sub scanning direction along the main scanning direction is disclosed in the Japanese Patent Application Laid-open No. 2002-148551. Further, another example in which the optical axis of the scanning lens is shifted with respect to the light beams to correct the displacement is disclosed in the Japanese Patent Application Laid-open No. H11-64758). Moreover, an example of a method of correcting the displacement using means such as inclining the scanning lens on a plane orthogonal to the scanned plane is disclosed in the Japanese Patent Application Laid-open No. S64-52116. In these examples, the degrees of the displacement among jobs are detected, and corrected depending on the degrees detected.
The deviations are now typically seen as more and more scanning lenses are being made of plastic. To prevent the deviations, the imaging unit formed of a scanning lens, which is common to the respective color beams and does not have a converging power in the sub scanning direction, and scanning lenses for the respective color beams, is disclosed for example in the Japanese Patent Application Laid-open Nos. H2-250020 and H7-43627. According to the imaging unit, even if the refracting power changes, the change is commonly shared by the respective color beams. As a result, the colors of the images formed become less out of registration. Since a plurality of light beams corresponding to the respective colors are scanned together by the same surface of the polygon mirror, optical means for gathering the light beams from the light source units and direct the light beams to be incident on the polygon mirror is preferably required as disclosed in the Japanese Patent Application Laid-open Nos. 2001-296492 and H9-179047.
For the arrangement of an optical system before the deflector in the tandem-type image formation apparatus, there is a method of decreasing width occupied by all the light beams in the sub scanning direction by defining the distances between the light beams lined in the sub scanning direction. For example, light beams from a plurality of light sources are superposed on each other in the main scanning direction by a turning mirror such that the light beams can be regarded substantially as a single light beam, and guided to an optical deflector. The turning mirror is placed at a distance in the direction of optical axis between an optical element that condenses the respective the bundle of rays to be substantially linearly long in the main scanning direction on the reflecting surface of the optical deflector, and the optical deflector. Accordingly, by changing the sizes of the reflecting surfaces of the respective mirrors in accordance with the diameters of the bundles of rays in the sub scanning direction on the respective mirrors, and making the distances between the adjacent light beams different, the width occupied by all the light beams in the sub scanning direction is decreased as disclosed in for example the Japanese Patent Publication No. 32222052.
In the multi-color image formation apparatuses in which a plurality of image forming stations are arranged along the transfer direction of the transfer member to superpose colors, if the positions of the latent images transferred by the respective stations do not accurately match each other, colors of the image formed may become out of registration or color change may be caused.
In the optical scanners, however, even if the deviation in the scanning position between the respective stations, which causes the colors to be out of registration, is adjusted before each job, if the number of sheets to be printed in one job increases, since the incident position on the scanning lens varies due to the deformation of the housing resulting from the temperature change, the variation caused until the next correction cannot be prevented,
Naturally, printing can be suspended to perform the correction in the middle of a job, but it takes time to correct the curving of the scan lines, in addition to the write start positions in the main scanning direction and the sub scanning direction of the scan lines. Further, it is necessary to record the detection patterns for detecting the deviations on the transfer member to detect the result of the correction. In the meantime, the apparatus cannot be used for the recording, waiting time for printing increases, and thus the efficiency of operation is decreased. Further, if the number of corrections increases, wasteful consumption of the toner increases. Therefore frequent corrections should be avoided.
The scanning position is preferably maintained stable despite of any environmental changes. Some of the above examples in which the common scanning lens is used for the respective color beams are directed to achieve this object. However, according to these examples, the thickness of the polygon mirror for deflecting the beams together is thick. Consequently, the edges of the polygon mirror generate a greater windage that increases the load and the power consumption. Therefore, it is desired to make the polygon mirror thin, and thus it is required to decrease the distances between the respective color beams. It is also necessary to place the optical system including to the light source units and the deflection unit together with respect to the deflection position of the single polygon mirror. However, to avoid interferences between these units, it is required to locate the units away from the polygon mirror. Accordingly, the size of the optical scanner cannot be downsized. It is therefore an object in the art to arrange the light source units such that space is spaced as much as possible.
For decreasing the distances between the respective light beams, the reflective mirror surface of the optical deflector may be decreased according to the above example disclosed by the Japanese Patent Publication No. 32222052. Another example with separating optical means is also disclosed in the Japanese Patent Application Laid-open No. 2000-330049, which includes light source means having a plurality of light emission sources and guides the light beams that have been deflected to the corresponding surfaces to be scanned. However, to separate the deflected light beams so as to be guided to the different surfaces, it is necessary to provide a refracting power in the sub scanning direction of the scanning lens shared by all the light beams, to increase the differences between the adjacent light beams in the sub scanning direction. When the scanning lens shared by all the light beams has the refracting power in the sub scanning direction, the shape of the main scanning cross section changes along the sub scanning direction, and when the incident bundles of rays are displaced in the sub scanning direction, uniformity of the velocities is decreased. The imaging performance in the main scanning direction also deteriorated. Further, when a temperature distribution occurs in the lens due to the generation of heat in the optical deflector, and the shape of the lens changes due to a uniform temperature change, “out of color registration” occurs due to the decrease in the uniformity of the velocities among the respective light beams.